1. Field of the Invention
The present invention relates to methods for making bottom-gate-type thin film transistors having active layers composed of polycrystalline silicon and the like. More particularly, the present invention relates to a technology for crystallization of a bottom-gate-type thin film transistor made by a low temperature process of 600.degree. C. or less.
2. Description of the Related Art
Thin film transistors suitable for switching elements in active matrix liquid crystal displays are being intensively developed. Polycrystalline or amorphous silicon is used as the active layer in thin film transistors. In particular, polycrystalline silicon thin film transistors have attracted attention because they enable the achievement of compact, high-definition, active matrix, color liquid crystal displays. The use of polycrystalline silicon having higher carrier mobility than that of amorphous silicon enhances current-driving characteristics of a thin film transistor; hence, a peripheral circuit section requiring high-speed driving can be formed together with thin film transistors for pixel switching on the same substrate.
In device and process technologies for thin film transistors, high temperature processes with process temperatures of higher than 1,000.degree. C. have been established. A high temperature process is characterized by modification of a semiconductor thin film formed on a highly heat-resistant substrate such as quartz by means of solid-phase deposition. In the solid-phase deposition, the semiconductor thin film is annealed at a temperature of higher than 1,000.degree. C. to grow individual microcrystal grains in polycrystalline silicon. The polycrystalline silicon formed by the solid-phase deposition has a high carrier mobility of approximately 100 cm.sup.2 /V-s. Since such a high temperature process essentially requires the use of a highly heat-resistant substrate, expensive quartz has been used. Quartz, however, has a disadvantage since it contributes to increased production costs.
Low temperature processes at temperatures of less than 600.degree. C. have been developed as a substitute for the high temperature process. Laser annealing using laser light has attracted attention as one of the low temperature processes for thin film transistors. In laser annealing, a non-single-crystal semiconductor thin film composed of amorphous or polycrystalline silicon deposited on a low heat-resistant insulating substrate of glass or the like is irradiated with laser light to locally melt the semiconductor thin film and then the semiconductor thin film is crystallized in the cooling step. Polycrystalline silicon thin film transistors are integrally formed by using the crystallized semiconductor thin film as an active layer (a channel region). Since the crystallized semiconductor thin film has a high carrier mobility, the resulting thin film transistors have excellent performance.
In laser annealing, a line-shaped laser beam (hereinafter referred to as a line beam) has been used. Line beams are scanned in a given direction on the semiconductor thin film, while partially overlapping the previously irradiated area. For example, short-duration pulses of line-shaped XeCl excimer laser light with a wavelength of 308 nm are repeatedly radiated. An exemplary line beam is shaped into a line of 300 mm by 0.5 mm, and has an irradiation energy density of 350 mJ/cm.sup.2. As an example, the pulse width of the line beam is approximately 40 nsec and the repetition frequency is approximately 150 Hz. The line beam pulses are radiated with an overlap rate of 90% to 99%.
Top-gate configurations are the mainstream of thin film transistors. In a top-gate configuration, a semiconductor thin film is deposited on an insulating substrate and a gate electrode is formed thereon with a gate insulating film formed therebetween. In a low temperature process, an inexpensive large glass substrate is used as the insulating substrate. The glass substrate contains large amounts of impurities such as sodium, which localize in response to a voltage for driving the thin film transistor. The electric field caused by the localization changes thin film transistor characteristics, resulting in deterioration of reliability. Recently, bottom-gate configurations suitable for low temperature processes have been developed as a countermeasure against the above-mentioned problem. In the bottom-gate configuration, a gate electrode of a metal film or the like is provided on an insulating substrate such as a glass plate, and a semiconductor thin film is formed thereon with a gate insulating film formed therebetween. The gate electrode shields the electric field in the glass plate; hence, the bottom-gate configuration is more reliable than the top-gate configuration.
The bottom-gate configuration, however, has a serious problem in crystallization by laser annealing. In the semiconductor thin film to be crystallized, the portion used primarily as a channel region lies just above the gate electrode, and the source and drain regions lie on the glass plate. Consequently, when energy by laser irradiation is applied, there are differences in thermal conduction and heat dissipation between the glass plate and the metal gate electrode. The channel region and the source and drain regions have, therefore, different optimum energies by laser irradiation, and optimum energy laser irradiation for achieving high carrier mobility is not possible. That is, in crystallization by laser annealing, both the semiconductor thin film on the metal gate electrode and the semiconductor thin film on the glass substrate are simultaneously irradiated with laser light. During cooling of the melt, the melt is solidified on the metal gate electrode within a relatively short time since the heat dissipates in the transverse direction through the gate line. Thus, the semiconductor thin film has different crystal grain sizes above the metal gate electrode and the glass substrate, and has nonuniform carrier mobility. In extreme cases, when an attempt is made to increase the crystal grain size of the semiconductor thin film on the metal gate electrode, the semiconductor thin film on the glass plate will vaporize because of the excessive amount of irradiated energy. In contrast, when an attempt is made to maintain the crystals of the semiconductor thin film on the glass plate in a normal state, the crystal grain size of the semiconductor thin film on the metal gate electrode is excessively reduced. Overlapping irradiation using line beams increases the possibility of the formation of fine holes by evaporation in the semiconductor thin film because of excessive laser irradiation energy. Line beam irradiation has a small range of allowable laser energy and it is difficult to determine the irradiation conditions.